The invention relates to apparatus for probing printed circuit (PC) boards to apply stimulus signals to selected points of the PC boards and to measure responses at other points of the PC boards to enable the responses to be compared with the expected response and thereby determine the operability of the PC board. The invention relates more particularly to fixtures that translate or conduct signals between conductive probes of a base grid having fixed center-to-center spacings and randomly oriented conductive test points of the PC board to be tested.
Typical state of the art printed circuit board testers include large numbers of programmable signal drivers for producing various stimulation signals to be applied to a printed circuit board under test, and also include a large number of programmable signal receivers for receiving signals produced by the printed circuit board under test in response to the stimulus signals, and further include means for comparing the response signals to expected responses. Typically, such a printed circuit board tester includes an array of conductors or pointed probe tips arranged in a rectangular array having 100 mil centers. When a particular printed circuit board is to be tested, it ordinarily will have a large number of probe points or test points which are randomly positioned on a surface of the printed circuit board. For unloaded printed circuit boards, such probe points often will be the locations of conductive feedthroughs. A fundamental problem encountered in the industry is how to reliably and economically provide connections between the various ones of the base grid conductors or probe points (which have been programmed to effectuate testing of a particular printed circuit board) and the randomly positioned probe or test points of the PC board to be tested. Use of translator boards that are "hand designed" to align conductive points of the base grid with the randomly positioned probe points of the PC board to be tested is excessively expensive, as there may be several thousand probes points on a particular PC board.
Automated techniques for translating or conducting signals between the base grid of the automatic tester and the PC board test points have been needed. However, what has been provided in the past are translating devices that utilize long, or solid spring-loaded probes, referred to herein as "probe pins", the lower ends of which contact signal pads or probes of the base grid. The upper ends of the probe pins contact a probe point of the printed circuit board to be tested. An upper guide plate having clearance holes therein aligned with the probe points of the printed circuit board to be tested is aligned with the printed board to be tested. A lower guide plate has clearance holes aligned with the conductors of the base grid. The spring-loaded probe pins then are dropped through holes in the upper guide plate and pass through holes in the bottom guide plate to make contact with various base grid conductors. The clearance holes are large enough to allow the spring-loaded probe pins to tilt sufficiently to ensure that they can pass through guide holes in both the upper and lower guide plates (since the guide holes in the upper and lower plate normally are not aligned due to the random placement of the guide holes in the upper plate).
A basic problem with some implementations of the foregoing prior art approach is that there is no precise way of determining exactly which conductors of the base grid make electrical contact 2 which probe points of the printed circuit board under test. Therefore, test algorithms which exercise the board under test (BUT) are inherently "non-deterministic". What this means is that circuit design data cannot be used to produce the expected response with which the actual response produced by the board under test in response to the stimulation is to be compared. Instead, various patterns of test stimuli must be applied to a particular printed circuit board that is known to be "good". The resulting response is stored and used as the basis of comparison with subsequently tested boards. Furthermore, the spring-loaded probe pins need to be quite long so that their angles of tilt or inclination, caused by misalignment of the randomly positioned PC board test points with the 100 mil centered base grid conductors, are not too great. As test point spacing decreases, the tilted probe pin approach becomes increasingly impractical.
Another problem with the foregoing approach is that a relatively large volume of air must exist between the base grid and the bottom of the printed circuit board to be tested. This large volume of air is rather incompatible with vacuum fixturing techniques wherein a vacuum is produced in the volume between a vacuum sealed pressure plate over the BUT and the base grid of the tester in order to force the BUT against the spring-loaded probe pin points.
It is clear that there remains an unmet need for an improved apparatus for translation of automatic tester base grid conductor signals to randomly positioned PC board test points.